The present invention is directed, in general, to power conversion and, more specifically, to a system and method for estimating magnetic flux in an isolation transformer and a power converter employing the system or the method.
A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunication and computer systems wherein high voltages are converted to lower voltages to operate the systems.
Current power converter designs often implement one of two full bridge control strategies, namely, the conventional (hard-switched) full bridge or the phase-shifted full bridge. Both control strategies employ a full bridge inverter topology having four controllable switches (e.g., power metal-oxide semiconductor field-effect transistors), an isolation transformer, an output rectifier and an output filter. A controller is included and employed to control the controllable switches.
The conventional full bridge generally operates as follows. The controllable switches are arranged in two diagonal pairs that are alternately turned on for a portion of a switching period to apply opposite polarities of the input DC voltage across a primary winding of the isolation transformer. The controllable switches thus operate to convert the input DC voltage into an AC voltage required to operate the isolation transformer. Between conduction intervals of the diagonal pairs, all of the controllable switches are turned off for a fraction of the switching period. Ideally, this should force a voltage across the primary winding of the isolation transformer to zero. The output rectifier then rectifies the AC voltage from the isolation transformer. A rectified voltage of the isolation transformer should, therefore, ideally be a square wave with an average value proportional to a duty ratio of the diagonal pairs of controllable switches.
The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the power converter. The controller monitors the output voltage and adjusts the duty ratio of the diagonal pairs of controllable switches to maintain the output voltage at a constant level as the input DC voltage and the load current vary.
The conventional full bridge suffers from a ringing between a leakage inductance of the isolation transformer and a parasitic capacitance of the controllable switches. The ringing dissipates energy, thereby reducing the efficiency of the power converter. The ringing also gives rise to significant noise, such as conducted and radiated electromagnetic interference.
The phase-shifted full bridge was developed to alleviate the switching loss and switching noise problems of the conventional full bridge. The construction of the phase-shifted full bridge is essentially identical to that of the conventional full bridge. Its advantages result, however, from the operation of the controllable switches to produce a zero voltage across the controllable switches before the controllable switches are turned on. The phase-shifted full bridge operates by turning off only one controllable switch of a diagonal pair to begin the zero voltage period, instead of turning off both of the controllable switches. A controllable switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate through the two controllable switches with substantially zero volts across the isolation transformer. The two controllable switches thus clamp the voltage across the isolation transformer at about zero, thereby substantially eliminating the ringing behavior suffered by the conventional full bridge when the controllable switches are off.
The magnetic flux in the isolation transformer should be sensed and controlled in both the conventional and the phase-shifted full bridge. A small imbalance in the duty cycles of the controllable switches or a small asymmetry in voltage drops across the controllable switches can result a volt-second imbalance between the two half-cycles of each switching cycle, which will result in a continuing cycle by cycle increase in the magnetic flux in the isolation transformer. A volt-second imbalance implies that a DC voltage component is applied to the core of the isolation transformer. Over a number of switching cycles, the increase in the magnetic flux may cause the core of the isolation transformer to saturate, resulting in failure of the power converter employing the isolation transformer.
A volt-second imbalance between the two half-cycles of each switching cycle is thus detrimental to the operation of the power converter. There are several causes of the volt-second imbalance, including an imbalance in the duty cycles of the controllable switches or a small asymmetry in the voltage drops across the controllable switches. Over a number of switching cycles, the continuing increase in the magnetic flux may cause the core of the isolation transformer to saturate. It is therefore necessary to estimate the magnetic flux in the isolation transformer and reduce the volt-second imbalance to avoid saturation of the core.
One common approach to estimating the magnetic flux in the isolation transformer is to employ a current sense transformer to directly sense the current in the primary winding of the isolation transformer. The controller may then operate the controllable switches to reduce the volt-second imbalance. Since ordinary (non-superconducting) transformers are unable to sense the DC voltage component, the above approach often requires two current sense transformers, each sensing either a positive or a negative current. Substantial expense and board real estate are thus required with the current sense transformer approach. Further, the current sense transformers will introduce a small parasitic inductance into the circuit that, when subjected to the rapid switching action of the controllable switches, may result in voltage transients and may necessitate the use of additional snubber circuitry.
Another common approach to estimating the magnetic flux in the isolation transformer is to employ a resistor series-coupled to the primary winding of the isolation transformer. A sensing circuit may then be employed to sense a voltage across the resistor to determine the current in the primary winding. The resistor, however, may dissipate a substantial amount of energy (especially in higher power applications), thereby reducing the efficiency of the power converter employing this approach.
Accordingly, what is needed in the art is a system and method for estimating magnetic flux in the isolation transformer that overcomes the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides, for use with a power converter having a full bridge switching circuit coupled across a primary winding of an isolation transformer, a system and method for estimating magnetic flux in the isolation transformer and a power converter employing the system or the method. In one embodiment, the system includes a conductive path, including an observer, established across at least a portion of the primary winding. A voltage developed by the observer depends upon a value of the magnetic flux in at least the portion of the primary winding.
The present invention, in one aspect, provides a system and method for estimating magnetic flux in an isolation transformer of a full bridge switching circuit. By estimating the magnetic flux in the isolation transformer, the controllable switches of the full bridge switching circuit may be adjusted to reduce a volt-second imbalance in the two half-cycles of each switching cycle, thereby avoiding failure of the power converter due to saturation of a core of the isolation transformer.
In one embodiment of the present invention, the magnetic flux is estimable without the use of a current sense transformer. As previously discussed, current sense transformers are often used in pairs and therefore may require substantial expense and board real estate. Additionally, the magnetic flux may also be estimable without the use of a resistor series-coupled to the primary winding of the isolation transformer. As previously discussed, the series-coupled resistor may dissipate a substantial amount of energy thereacross, thereby reducing the efficiency of the power converter.
In one embodiment of the present invention, the observer includes a series-coupled sense capacitor and sense resistor. The magnetic flux in the primary winding may be estimated by measuring a voltage across the capacitor. In an alternative embodiment, the observer includes a series-coupled sense resistor and sense inductor. The magnetic flux in the primary winding may be estimated by measuring a voltage across the resistor. In either case, the observer allows the magnetic flux in the primary winding to be estimated and controlled.
In one embodiment of the present invention, the full bridge switching circuit is operable in a phase-shifted mode. In another embodiment, the full bridge switching circuit is operable in a conventional or hard-switched mode. Those skilled in the art are familiar with both the phase-shifted and hard-switched modes of operating the full bridge switching circuit.
In one embodiment of the present invention, the power converter employs the system to reduce an imbalance of the magnetic flux in the isolation transformer. Saturation of the isolation transformer and resultant failure of the power converter may thus be avoided.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.